This invention relates to variable-capacitance position transducing. This is a continuation-in-part of U.S. patent application Ser. No. 07/585,890.
Variable-capacitance transducers typically include a stationary portion, such as a pair of capacitor plates, that is held as one frame of reference and a moving portion, such as a rotating dielectric located between the capacitor plates, that is held as the second frame of reference. Generally, such position sensors may be as accurate as a part in a thousand or even a part in a hundred thousand.
Variable-capacitance transducers are used primarily in closed-loop systems for precise positioning of rotatable elements of various mechanisms such as galvanometers, stepping motors, gimbals, gyros, and similar devices. In closed-loop systems, errors that are the result of motor and galvanometer non-linearities or external forces are monitored and corrected in order to improve accuracy of positioning. Position transducers in closed-loop systems produce information that is used to compensate for mechanical properties of motors, galvanometers, and other devices that could not be corrected in open-loop systems. Such transducers can alternatively be used in open-loop systems as a reporting system to report the orientation of a rotating member.
The desired movement of the rotatable element may be oscillatory or step-and-settle in nature. Variable-capacitance position transducers are also useful in holding rotatable elements accurately in a fixed position. For example, a rotating shaft may be used to deflect a mirror to a position at which a laser is incident on the mirror and at which the reflected beam in turn works upon a material or object, as occurs in microlithography, laser trimming of resistors in film resistor applications, repairing of memory circuits in silicon memory chips prior to packaging of the chips in final form, exposure of photosensitive films or papers, reprographic applications, or scanning of a field to report video information.
In a known variable-capacitance transducer, shown in FIG. 10, four electrode segments 82, 83, 84, and 85 are arranged cylindrically around a rotor 86. The four electrode segments are constructed as two pairs of segments, the two electrode segments in each pair being interconnected by a conductive ring 87 or 88. A conductive outer cylinder, consisting of guard plates 89 and 90 that attach to plate 1, is constructed around rings 87 and 88. The outer cylinder shields conductive rings 87 and 88 from stray capacitances with respect to external structures.